The atomic nuclei of certain elements exhibit angular momentum and an associated magnetic moment. When such nuclei are placed in a magnetic field they can adopt a number of quantized orientations, each orientation corresponding to a particular energy level. Nuclear magnetic resonance (NMR) involves transitions between these energy levels. The transitions may be induced by the absorption of radiation of a particular frequency. The absorption or emission of radiation may be detected to obtain a nuclear magnetic resonance spectrum of samples containing the certain elements.
Spatial information regarding the location of a region of interest is conventionally obtained using MRI by superimposing static magnetic field gradients, i.e. magnetic fields which change linearly in a given direction. The two dimensions perpendicular to each gradient field remain unresolved and one dimensional projection of the three dimensional sample are obtained. An image of the sample is mathematically reconstructed from the one dimensional projections. MRI has found widespread acceptance as a medical diagnostic technique for providing images of internal soft tissue structures of the human body. However, it must be recognized that the static magnetic field gradients that are essential to conventional imaging techniques make it impossible to resolve the NMR chemical spectrum of any compound in the volume under study. Simply put, the position of an NMR line which serves to identify the element in terms of a shift of so may ppm (parts per million) of magnetic field strength could be due to either a chemical shift of that element or the fact that the magnetic field has indeed changed by that amount due to the imposed magnetic field gradient employed by the imaging scheme.